There is a high interest in wireless communication to develop highly linear and efficient power amplifier suitable for third generation (3G) and upcoming fourth generation (4G) of communication standards. These new standards have potential for offering more and better data services. However, in order to establish the highly linear and efficient power amplifier within a restricted frequency band, it is needed to use signals with high peak-to-average power ratios (e.g., wideband code division multiple access (WCDMA)˜10 dB), requiring high linearity of the transmitting amplifier. Hence, in general, power amplifiers are designed to be subjected to the peak-power condition, but are operated most of the time at sufficiently lower power levels (power peak-off). As a result, the power amplifier operating with these signals may function such that the peak efficiency is high, but the average amplifier efficiency is rather low.
For this reason, there is a renewed interest in high efficiency power amplifiers, for example the Doherty power amplifier (DPA), linear amplification using nonlinear components (LINC), envelope tracking (ET) and the like. These types of the amplifiers are currently investigated as potential candidates suitable for the current and above-mentioned upcoming communication standards. In such a radio frequency (RF) amplifier demands more high efficiency and high linearity than general RF amplifier. Specifically, due to its improved efficiency, low complexity, and low cost, the two-way DPA has already penetrated the market. The DPA is, in general, high efficiency than general balanced amplifier. Although a general designs focus only amplitude distortions and phase distortions, a memory effect and linearity become very important factors recently. Such a general structure of the DPA has a weak point which leads high distortion such as third and fifth inter-modulation distortions (IMDs). For example, the distortion results from a sudden variation of impedance at signal synthesized point. To reduce the distortion by structure of the DPA, it demands complex compensator. The memory effect includes electric memory effect and electrothermal memory effect. The electric memory effect is occurred by bias and matching circuit's impedance variation in base and harmonic band. The electrothermal memory effect is FET power amplifier's gain variation by device temperature. The electrothermal memory effect is inevitable factor.
Therefore, proper compensator structures, such as the composite high power amplifier (C-HPA) have been introduced to increase the efficiency and output power. Such C-HPA comprises several individual high power amplifiers (HPAs). Most common configuration is C-HPA with two HPAs which also known as a LInear amplification using Nonlinear Components (LINC).
The LINC technique was first proposed in 1974 as a method of achieving linear amplification at microwave frequencies. The basic scheme of LINC for amplifier has two RF HPAs that are assumed to be high-efficiency and highly nonlinear. The RF source signal x(t) is split into two constant envelope, phase modulated signals, x1(t), x2(t) by signal component separator (SCS) that has a function of the signal separation or generation process, and each is fed into its own nonlinear RF power amplifier. The HPAs separately increase the power of each signal to generate output signals y1(t) and y2(t) before feeding them into a summing junction for recombination. The resulting output signal from the summing junction is then an amplified version of the original input signal without any distortion, if all components constituting the HPA are ideal ones.
In many issues it is assumed that the bandwidth at the digital-to-analog convertor (DAC) output is unlimited. However for many cases such an assumption is not applicable. Normally DACs have the low-pass filter (LPF) at its output. Such LPFs introduce some parasitic amplitude modulation (AM) into the DAC output signal. With such parasitic AM and non-linear HPA the complete transmitted signal reconstruction is not possible. Thus the out-of-band spectrum components are arising in the HPA output signal spectrum.
Despite the original signal x(t) has a narrow spectrum, the signal component separating (SCS) operation causes a significant spectrum expansion for signals x1(t) and x2(t) in the C-HPA arms. In the conventional technique disclosed in, for example, patent document 1 and non-patent documents 1-4 cited above, it is assumed the ideal arms with unlimited bandwidth (i.e. ultra-wideband ideal DACs) that may pass wideband signals x1(t) and x2(t) without any distortions to inputs to the HPAs (hereinafter, sometimes to be referred to as HPA1 and HPA2). The most critical element in the arm may be DAC because its bandwidth is limited by operating clock. Typically this is 100-150 MHz for low/mid-ends commercial available Large Scale Integrations (LSIs). In contrast, DACs for the broadband applications such as Worldwide interoperability for Microwave Access (WiMAX), Long Term Evoluation (LTE), Wideband Code Division Multiple Access (W-CDMA) and the like, in order to avoid distortions in signals x1(t) and x2(t), must provide very high bandwidth (must operate with high clock frequencies in order several hundred MHz) and at the same time provide a high level of bit resolution (quantization), for example, 800 MHz clock and 14 bit resolution. The cost for such a hi-speed and hi-resolution hi-end DACs is high.
The frequency restrictions caused by component of a high frequency circuit such as LPFs at the DACs inputs (or DAC outputs) result in the parasitic AM at the HAP1 and HAP2 inputs for signals x1(t) and x2(t). The LPFs cut some high frequency part of signals x1(t) and x2(t). The high frequency part that cut by LPF causes the unwanted parasitic AM modulation in signals at the LPF output. Such parasitic AM results in growth of out-of-band spectrum components after combining signals y1 and y2. In general, the perfect signal reconstruction after combining signals passed through LPFs becomes difficult.
The problem arises when RF signal is amplified with the C-HPA that comprises two or more individual arms, each of the arms has high power amplifier (HPA) having the bandwidth restriction. Such frequency restriction may be attributed to frequency characteristics of LPF, low-speed DAC, and/or narrow-band quadrature modulators.
One of the aims of the present invention is to provide a modulator that removes parasitic amplitude modulation automatically, thereby reconstructing a signal inputted into the modulator at an output terminal of the LINC modulator.
Another of the aims of the present invention is to provide am amplifier having a modulator that removes parasitic amplitude modulation automatically, thereby reconstructing a signal inputted into the modulator at an output terminal of the LINC modulator.